1. Field of the Invention
The present disclosure relates generally to wellbore imaging, and particularly to a performance verification of an instrument useful for resistivity imaging.
2. Background of the Related Art
Imaging of subsurface materials is of vital importance in the exploration for petro-chemical materials. A number of technologies may be used with a great variety of techniques for employing each of these technologies. Generally, each technique provides a particular advantage for a given situation. One prominent technology is imaging of resistivity of the subsurface materials.
First, and for perspective, consider that imaging of subsurface materials is typically conducted by deploying an imaging instrument in a borehole (also referred to as a “wellbore”) which has been drilled into the earth. Often, the process of drilling calls for introducing drilling mud into the borehole. The drilling mud provides a number of benefits, including a restraint against uncontrolled relief of pressure (and hydrocarbons) from downhole. However, use of some formulations of drilling mud, such as those that are oil based mud (OBM) can pose certain problems when attempting to image the subsurface materials.
As known to those skilled in the art of resistivity imaging, there are two major classes of instruments. First, “induction” instruments generally provide primary excitation by a magnetic field generated by an induction transmitter. The second class, “galvanic” instruments use a set of electrodes or electrical dipoles to generate an electrical field due to a potential between them and thus drive currents into the materials surrounding the instrument.
A vast majority of induction instruments operate in a relatively low frequency range. This results in instrument sensors having minimal electromagnetic coupling with the surroundings which results in a small power efficiency of measurement. Advantageously, however, this allows for keeping sensor parameters such as magnetic moments, transfer functions, and characteristic impedances almost unchanged during logging. Using these features of induction instruments, methods have been established for proper instrument verification and calibration, both in the laboratory and at a wellsite.
In contrast, proper instrument verification and calibration for galvanic tools is generally unavailable. This is even more problematic as a new class of high frequency galvanic instruments is proving to be useful for evaluation of sub-surface materials. That is, use of oil based mud (OBM) that is commonly used in drilling, interferes with resistivity measurements. Accordingly, the resistivity effects of the OBM must be accounted for or overcome to properly ascertain properties of the surrounding sub-surface materials.
Attempts to discount effects of OBM on resistivity data have generally relied upon calibration of resistivity instruments on the surface, such as in a laboratory. However, as downhole environments are complex and harsh (for example, such environments often exhibit high temperature and high pressure), calibration completed on the surface is of limited use. More specifically, it has been found that many factors play a role in the performance of the instrument downhole. Accordingly, determinations of instrument performance conducted on the surface are of limited value.
Accordingly, there is a need for in-situ performance verification and/or calibration of a high frequency imaging instrument.